Many metals pose a risk as environmental contaminants. A well-known example is lead. Low level lead exposure can lead to a number of adverse health effects, with as many as 9-25% of pre-school children presently at risk. Approximately twenty-two million old houses in the United States alone have lead paint (Schwartz & Levin, 1991; Rabinowitz et al., 1985). Although leaded paints and gasoline have been banned, lead can accumulate in soils or sediments for long periods of time (Marcus & Elias, 1995; Bogden & Louria, 1975). The level of lead in the blood considered toxic is ≧10 μg/dL (480 nM). Current methods for lead analysis, such as atomic absorption spectrometry, inductively coupled plasma mass spectrometry, and anodic stripping voltammetry, are complex, expensive and often require sophisticated equipment, sample pre-treatment and skilled operators.
Simple, rapid, inexpensive, selective and sensitive methods that permit real time detection of Pb2+ and other metal ions are very important in the fields of environmental monitoring, clinical toxicology, wastewater treatment, and industrial process monitoring and can lead to preventative measures or at least lower risks associated with metal contaminants. Furthermore, methods are needed for monitoring free or bioavailable, instead of total, metal ions in industrial and biological systems.
Fluorescence spectroscopy is a technique well suited for detection of very small concentrations of analytes. Fluorescence provides significant signal amplification, since a single fluorophore can absorb and emit many photons, leading to strong signals even at very low concentrations. In addition, the fluorescence time-scale is fast enough to allow real-time monitoring of concentration fluctuations. Fluorescent properties only respond to changes related to the fluorophore, and therefore can be highly selective. Also, fluorometers, for measuring fluorescence signals, are commercially available. Fluorescent detection is also compatible with fiber-optic technology and well suited for in vivo imaging applications. Several fluorescence-related parameters can be assessed for purposes of sensing, detecting, identifying or quantifying a target analyte, including fluorescence intensity, emission or excitation wavelength, fluorescence lifetime and anisotropy.
For example, bioaffinity sensors, labeled with fluorophores, have been used to detect DNA hybridization and single-nucleotide polymorphisms (Didenko, 2001). Specifically, molecular beacon, a DNA hairpin structure, is labeled with both a fluorophore and quencher (Tyagi & Kramer, 1996). In the absence of target DNA, the hairpin structure is closed and due to the close proximity of the fluorophore and quencher, fluorescence is quenched. However, in the presence of a complementary DNA strand, the hairpin secondary structure is destroyed and the fluorescence is released without quenching. Multiple DNA strands may be detected at the same time by placing a quencher on one end of the molecular beacon DNA strand and two fluorophores (a donor fluorophore and an acceptor fluorophore) on the other end (Tyagi & Kramer, 1998; 2000). This design, based on fluorescence resonance energy transfer (FRET), quenches fluorescence of the fluorophores in the absence of complementary DNA due to the hairpin structure being closed. However, upon hybridization of the molecular beacon and the complementary DNA, the secondary structure is destroyed and the donor fluorophore transfers energy to the acceptor fluorophore, resulting in fluorescence. Molecular beacon can be designed to target different DNA sequences by constructing complementary DNA strand hairpins, each with a different acceptor fluorophore, while keeping the donor fluorophore the same.
Biosensors, devices capable of detecting target ions using biological reactions, in contrast to bioaffinity sensors, can be modified to utilize fluorescence for detecting, identifying or quantifying target ions, which can act as catalysts of the biosensor. These modified biosensors, called fluorosensors, are highly sensitive. For example, many fluorescent chemosensors, including fluorophore-labeled organic chelators (Rurack, et al., 2000; Hennrich et al., 1999; Winkler et al., 1998; Oehme & Wolfbeis, 1997) and peptides (Walkup & Imperiali, 1996; Deo & Godwin, 2000; Pearce et al., 1998), have been developed for metal ion detection. These ion sensors are usually composed of an ion-binding motif and a fluorophore. Metal detection using these fluorescent chemosensors relies on the modulation of the fluorescent properties of the fluorophore by the metal-binding event. Detection limits on the level of micromolar and even nanomolar concentrations have been achieved for heavy metal ions including Zn2+, Cu2+, Hg2+, Cd2+ and Ag+.
Recently, the molecular recognition and catalytic function of nucleic acids have been extensively explored. This exploration has led to the development of aptamers and nucleic acid enzymes, which can be used as biosensors. Aptamers are single-stranded oligonucleotides derived from an in vitro evolution protocol called systematic evolution of ligands by exponential enrichment (SELEX). Nucleic acid aptamers can selectively bind to non-nucleic acid targets, such as small organic molecules or proteins, with affinities as high as 10−14 M (Uphoff et al., 1996; Famulok, 1999). Most aptamers undergo a conformational change when binding their cognate ligands. With this property, several DNA and RNA aptamers have been engineered to sense L-adenosine or thrombin through an internally labeled fluorescent reporter group (Jhaveri et al., 2000). Thus, the conformational change in the aptamer upon binding leads to a change in fluorescence. Nucleic acid enzymes, molecules capable of catalyzing a chemical reaction, may be specifically designed through in vitro selection. (Breaker & Joyce, 1994; Breaker, 1997). Allosteric ribozymes (or aptazymes), which combine the features of both aptamer and catalytic RNA, also hold promise for sensing small molecules (Potyrailo et al., 1998; Koizumi et al., 1999; Robertson & Ellington, 1999, 2000). Their reactivity is modulated through the conformational changes caused by the binding of small organic molecules to an allosteric aptamer domain. Therefore, the signal of ligand binding can be transformed into a signal related to chemical reaction.